Magnetic discs and disc drives are employed for storing data in magnetizable form. Preferably, one or more discs are rotated on a central axis in combination with data transducing heads positioned in close proximity to the recording surfaces of the discs and moved generally radially with respect thereto. Magnetic discs are usually housed in a magnetic disc unit in a stationary state with a magnetic head having a specific load flying over the surface of the disc. Data are written onto and read from a rapidly rotating recording disc by means of a magnetic head transducer assembly that flies closely over the surface of the rotating disc. Preferably, each face of each disc will have its own independent head. The transducer assembly must be held very closely spaced from the rotating disc surface as a condition of achieving high information storage density.
FIG. 1 is an exploded perspective view of a disc drive storage system in which the present disc would be useful. Clearly, the disc is not limited to use in such disc drives. In fact, although a magnetic disc drive is shown, the disc could equally well be used in a magneto-optical disc drive. In fact, plastic disc substrates would probably be especially useful in removable storage applications, such as CD-ROM or magneto-optical drives. Thus, FIG. 1 is provided primarily to give an illustrative example of the environment in which a rotating hard storage disc is used.
In this particular example of FIG. 1, the storage system 10 includes a housing base 12 having a spindle motor 14 which rotatably carries the storage discs 16 which are to be discussed in detail below. An armature assembly 18 moves transducers 20 across the surface of the discs 16. The environment of disc 16 is sealed by seal 22 and cover 24. In operation, discs 16 rotate at high speed while transducers 20 are positioned at any one of a set of radially differentiated tracks on the surface of the disc 16. This allows the transducers 20 to read and write encoded information on the surface of the discs at selected locations. The discs rotate at very high speed, several thousand RPM, in order to maintain each transducer flying over the surface of the associated disc. In present day technology, the spacing distance between the transducer and the rotating disc surface is measured in micro inches; thus it is absolutely essential that the disc does not vibrate while it is being rotated, as such vibration could easily disturb the air flow which is maintaining the flight of the transducer over the surface, or simply cause mechanical contact between the transducer and the disc surface. Such contact would probably damage the disc surface, resulting in the loss of disc storage space; it could even damage the transducer, resulting in loss of use of the disc drive.
A disc recording medium is shown in FIG. 2. Even though FIG. 2 shows sequential layers of magnetic media on one side of the non-magnetic substrate 2-20, it could be sputter deposited sequential layers on both sides of the non-magnetic substrate.
Adverting to FIG. 2, an adhesive sub-seed layer 2-21 is deposited on substrate 2-20, e.g., a glass or glass-ceramic, Al or AlMg substrate. Subsequently, a seed layer 2-22 is deposited on the sub-seed layer 2-21. Then, an underlayer 2-23, is sputter deposited on the seed layer 2-22. An intermediate or flash layer 2-24 is then sputter deposited on underlayer 2-23. Magnetic layer 2-25 is then sputter deposited on the intermediate layer, e.g., CoCrPtTa. A protective covering overcoat 2-26 is then sputter deposited on the magnetic layer 2-25. A lubricant topcoat (not shown in FIG. 2 for illustrative convenience) is deposited on the protective covering overcoat 2-26.
The disc is finely balanced and finished to microscopic tolerance. Take the smoothness of its surface, for example. The drive head rides a cushion of air at microscopic distances above the surface of the disc. So, the surface cannot be too smooth, or the drive head will end up sticking to the disc, and it cannot be too rough either, or the head will end up getting caught in the microscopic bumps on the surface.
It is considered desirable during reading and recording operations to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. This objective becomes particularly significant as the areal recording density and drive speed increase. The areal density (Gbits/in2) is the recording density per unit area and is equal to the track density (TPI) in terms of tracks per inch times the linear density (BPI) in terms of bits per inch.
As areal density and drive speed increase, excessive surface roughness of the substrate (at a microscopic level) can cause head crash due to accidental glide hits of the head and media. To minimize head crash due to accidental glide hits, conventional techniques of manufacturing magnetic recording media produce a smooth surface on the disc by polishing the substrate prior to sputter and tape burnishing (buffing) and tape wiping the media. See, for example, U.S. Ser. No. 10/662,426, Nakamura et al., U.S. Pat. No. 5,202,810 and Bornhorst et al., U.S. Pat. No. 4,430,782. Typically, the substrate polishing is done using slurry and buffing/wiping is done after sputtering. However, these conventional techniques are attendant with numerous disadvantages for plastic substrate media. For example, a lack of strong adhesion of the plastic substrate and magnetic media, including the magnetic layer(s), could cause a separation of magnetic media from the substrate during buffing or wiping.
To avoid glide hits, a smooth defect-free surface in the data zone is desired. The direct result of these demands is tending towards low yield due to less defect tolerance at the surface of the media. Thus, it is desired to provide an improved adhesion between the plastic substrate and magnetic media deposited thereon to allow for burnishing/polishing the surface of the media having plastic disc substrates to produce defect-free surfaces.